Conventionally, there has been an FET having a structure formed by a metal film, a ferroelectric film and a semiconductor layer (hereinafter referred to as an MFS structure) as a typical memory (semiconductor memory device) that is adapted to detect a resistance change in the semiconductor layer due to residual polarization in the ferroelectric film. This FET uses a ferroelectric material for a gate dielectric film. As shown in FIGS. 4(a)-(b) with residual polarization, an inversion layer is created in the channel region by ferroelectric residual polarization, enabling write-in. The memory of this type can perform reading out in a non-destructive manner, being advantageous in increasing the rewritable cycle life. In FIGS. 4(a)-(b), 21 is a semiconductor substrate, for example, of a p-type. 22, 23 are respectively source and drain regions formed by introducing an n.sup.+ impurity. 26 is a channel region sandwiched between the source region 22 and the drain region 23. The channel region 26 is formed thereon with a ferroelectric film 27 and a gate electrode 28. FIG. 4(a) illustrates an ON state that the gate electrode 28 is applied with a positive potential, while FIG. 4(b) shows an OFF state that the gate electrode 28 is applied with a negative potential. This ferroelectric film 27 conventionally has an oxide perovskite structure such as of BaTiO.sub.3, PZT (Pb(Zr.sub.1-x Ti.sub.x)O.sub.3), PLZT (Pb.sub.1-y La.sub.y (Zr.sub.1-a Ti.sub.a) .sub.1-y/4 O.sub.3).
In the MFS structure, however, when a ferroelectric film 27 is formed on an Si semiconductor substrate 21, an unwanted film such as of SiO.sub.2 is formed at an interface of between them. This causes not only increase in operating voltage but also injection of electric charges by appearance of trap into the ferroelectric film 27. This in turns causes that electric charges are canceled by t he residual polarity. In order to avoid this problem, considerations are being given to a MFMIS structure having overlying layers of, from the above, a control electrode, a ferroelectric film, a floating gate, a gate oxide (SiO.sub.2) , and a Si substrate. This structure enables the ferroelectric material to be film-formed on an electrode metal material so that a ferroelectric film can be formed with proper matching onto the electrode by selecting a metal material.
If an oxide of the oxide-perovskite structure, other than REMNO.sub.3, is used as a ferroelectric material as the conventionally done, the Si substrate on which a dielectric film is to be directly formed will have an oxide film on the surface due to oxidation. This oxide film is low in dielectric constant and consumes a voltage much more than the ferroelectric film having a greater dielectric constant, raising a problem of requiring high write-in voltage. Further, there is a possibility that oxygen deficiency occurs in the conventionally-used oxide perovskite structured dielectric, resulting in change of valence number and hence increase of space charges. This raises a problem of lower in ferroelectric characteristics.
Meanwhile, the present inventors has proposed a use of REMNO.sub.3 material for nonvolatile memories as disclosed in "Proposal of REMNO.sub.3 Thin Film to Nonvolatile Memories" in 56th Applied Physics Academy Study Lecture Preliminary Paper, page 440 (published on Aug. 26, 1995), wherein the same material is an oxide of lanthanoid group elements RE including Y and Mn , and has an advantage of possessing a dielectric characteristic and a small dielectric constant. However, REMNO.sub.3 is difficult to determine a film-forming condition for forming a complete crystalline structure. Therefore, it is poor in dielectric characteristic such as of leak current and hence not placed in practical applications.
The present invention has been made in view of such circumstances, and it is an object to provide a ferroelectric material that has a basic REMNO.sub.3 structure having improved dielectric characteristics and excellent crystallinity, wherein where used for a semiconductor memory or the like the characteristics thereof can be improved.
It is another object of the present invention to provide a concrete film-forming method using a ferroelectric film by which it is possible to form a ferroelectric film having REMNO.sub.3 thus improved in ferroelectric characteristics on a semiconductor substrate or the like.
It is further object of the present invention to provide a semiconductor memory device utilizing the inventive ferroelectric material and a method of manufacturing the same.